WDM systems involve a number of channels of different wavelength being transmitted simultaneously in an optical fibre, each channel typically being defined by a laser, or high brightness infra-red light emitting diode (IR LED) source. This technique permits the transmission capability of the optical fibre to be upgraded to the multi-Gbit/s range. Tunable optical devices such as tunable optical filters are used in such systems to select a particular wavelength from a range of input wavelengths, with the ability to alter the wavelength to be selected by applying an electrical signal to the filter.
Tunable optical filters are well known. Reference is directed to J S Patel: "Electro-optic Switch Using a Liquid Crystal Fabry-Perot Filter", Proceedings of the SPIE, Vol. 1665, pp. 244-249 (1992). Present methods of making these include a Fabry-Perot resonant cavity containing a discrete liquid crystal cell (discrete cell LCFP), in which a thin layer of liquid crystal material is confined between glass plates with broad band reflective coatings. Reference is directed to A. Sneh, K. M. Johnson, J. Y. Liu: "High-Speed Wavelength Tunable Liquid Crystal Filter", IEEE Phot. Tech. Letters, Vol. 7 No. 4, April 1995, pp. 379-381.
Japanese Laid-Open Patent Publication [1988] 63-5327, "Fabry-Perot Resonator", dispenses with the need for a discrete liquid crystal filled cell and discloses a cell enclosing opposing optical fibres, with the space between and around the ends of the fibres being filled with a liquid crystal material. Electrodes are attached to electrically conductive mirror coatings on the ends of the fibres.